<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
    .box1
    {
        width: 200px;
        height: 200px;
        background-color: yellow;
        /*float: left;*/
        overflow: hidden;
    }
    .box2
    {
        width: 200px;
        height: 200px;
        background-color: red;
        overflow: hidden;
    }
    .box3
    {
        width: 100px;
        height: 100px;
        background-color: blue;
        margin-top: 100px;
    }
    </style>
</head>
<body>
    <div class="box1">
        <div class="box3"></div>
    </div>
    <!--<div class="box2"></div>-->
</body>
</html>